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5.0 Requirements for Stormwater Management

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5.0 Requirements for Stormwater Management WATERSHED MANAGEMENT ORDINANCE TECHNICAL GUIDANCE MANUAL 5.0 REQUIREMENTS FOR STORMWATER MANAGEMENT 5.0.1 ARTICLE SUMMARY When development occurs, the natural or existing conveyance and storage capacity of land is reduced or even eliminated, and the resulting stormwater runoff can adversely impact adjacent and downstream properties by increasing flood elevations or decreasing flood conveyance capacity. The goal of stormwater management is to minimize the potential of these adverse impacts. To achieve this goal, stormwater management must be designed and incorporated into the development to ensure the resulting stormwater runoff does not increase flooding. Stormwater management includes controlling of the rate and volume of stormwater runoff. The WMO requires several stormwater management practices to be incorporated into the development to mitigate potential flooding impacts including: • Runoff • Volume control • Detention The WMO establishes the following standards for development and stormwater management: • General Development and Stormwater Management Requirements (§501) • Runoff Requirements (§502) • Volume Control Requirements (§503) • Detention Requirements (§504) • Development and Redevelopment Tributary to Existing Detention Facilities (§505) This section of the TGM provides guidance on performing a hydrologic and hydraulic analysis to calculate the quantity of stormwater runoff from a development, and design guidelines for a stormwater management facility to comply with the WMO stormwater management requirements. Detailed examples are provided at the end of each section. NOTE: All bold words are defined in Appendix A of the WMO and the TGM. 10/7/2019 5.0 REQUIREMENTS FOR STORMWATER MANAGEMENT PAGE 5-1 WATERSHED MANAGEMENT ORDINANCE TECHNICAL GUIDANCE MANUAL 5.1 GENERAL DEVELOPMENT AND STORMWATER MANAGEMENT REQUIREMENTS 5.1.1 GENERAL DEVELOPMENT REQUIREMENTS §501.1 of the WMO establishes the following standards to ensure stormwater runoff from development does not: • Increase flood elevations or decrease flood conveyance capacity upstream or downstream of the property holdings; • Pose any increase in flood velocity or impairment of the hydrologic and hydraulic functions of streams and floodplains unless a water resource benefit is realized; • Unreasonably or unnecessarily degrade surface or groundwater quality; or • Result in any new or additional expense to any person other than the applicant as a result of stormwater discharge. 5.1.2 GENERAL STORMWATER MANAGEMENT REQUIREMENTS §501.2 of the WMO regulates stormwater management based on the type of development and the size of the property holdings. (Table 2 in §501.2 of the WMO) summarizes the applicable stormwater management requirements. It is important to note the following items for Table 5.1: • Single-Family Homes are exempt from all stormwater management requirements. • Stormwater management requirements do not apply to demolition or maintenance activities. • Volume control and detention must be provided for right-of-way development when the new impervious area is greater than or equal to 1.0-acre where practicable. These types of development are often limited by public right-of-way constraints. If the stormwater management requirements are not provided in full, the applicant must demonstrate the right-of-way development has complied with the stormwater management requirements to the maximum extent possible. 10/7/2019 5.1 GENERAL DEVELOPMENT AND STORMWATER MANAGEMENT REQUIREMENTS PAGE 5-2 WATERSHED MANAGEMENT ORDINANCE TECHNICAL GUIDANCE MANUAL TABLE 5.1 SUMMARY OF STORMWATER MANAGEMENT REQUIREMENTS §502 §503 §504 Development Type Runoff Volume Control Detention (See Appendix A for definitions) Requirements1,2 Requirements1,2 Requirements1,2 Single-Family Home Exempt Exempt Exempt Residential Subdivision ≥ 1 acre ≥ 1 acre ≥ 5 acres on property holdings Multi-Family Residential ≥ 0.5 acre ≥ 0.5 acre ≥ 3 acres‡ on property holdings Non-Residential ≥ 0.5 acre ≥ 0.5 acre ≥ 3 acres‡ on property holdings Open Space ≥ 0.5 acre Not Applicable Not Applicable on property holdings Right-of-Way ≥ 1 acre ≥ 1 acre† ≥ 1 acre† when new impervious area 1 Stormwater management requirements do not apply to demolition or maintenance activities. 2 Requirements are applicable when a Watershed Management Permit is required under §201 of the WMO. † Where practicable. ‡ Starting the effective date of the WMO, any new development within the property holdings that totals either individually or in the aggregate to greater than or equal to one-half (0.5) of an acre. §501.3 of the WMO allows development that incorporates in-kind replacement with green infrastructure to be considered non-qualified development. For this provision, a volume control practice used to comply with the volume control requirements of the WMO is not considered green infrastructure. Therefore, the non-qualified development area can only be excluded from the detention requirements of the WMO when green infrastructure is not used to comply with the volume control requirements of the WMO. The following examples are provided: • A development that replaces a traditional paved parking lot with permeable pavers can be considered non-qualified development provided the permeable paver system is not used to comply with the volume control requirements of the WMO. • A development proposes a building, sanitary sewer, and permeable paver parking lot on a property that has never been developed. The permeable paver system is not considered non-qualified development since it is a new parking lot and not in-kind replacement. • A development replaces an existing building with a new building that includes a green roof. The building replacement is not considered non-qualified development since the WMO does not consider any building replacement as in-kind replacement. 10/7/2019 5.1 GENERAL DEVELOPMENT AND STORMWATER MANAGEMENT REQUIREMENTS PAGE 5-3 WATERSHED MANAGEMENT ORDINANCE TECHNICAL GUIDANCE MANUAL §501.4 of the WMO allows, under certain circumstances, a Watershed Management Permit to be issued without the applicant providing detention for the undeveloped area within the property holdings. This situation typically occurs when the development only takes place on a small portion of the larger undeveloped property holdings. If detention is only provided for the development, and not the entire property holdings, then the following items apply: • The applicant must submit Schedule L along with a plat of survey (Exhibit A) for the property holdings. Exhibit A must include a legal description and include all PINs. Schedule L and Exhibit A will be recorded with the Cook County Recorder of Deeds as an encumbrance against the entire parcel; and • A special condition, requiring detention to be provided for any future development within the property holdings, will be made a part of the Watershed Management Permit. 10/7/2019 5.1 GENERAL DEVELOPMENT AND STORMWATER MANAGEMENT REQUIREMENTS PAGE 5-4 WATERSHED MANAGEMENT ORDINANCE TECHNICAL GUIDANCE MANUAL 5.2 RUNOFF REQUIREMENTS 5.2.1 GENERAL RUNOFF REQUIREMENTS §502.1 of the WMO regulates runoff based on the type of development and the size of the property holdings being developed. Table 5.1 (refer to 5.1.2) summarizes when runoff requirements are applicable. It is important to note the following items: • Single-Family Homes are exempt from runoff requirements. • Stormwater management requirements do not apply to demolition or maintenance activities. 5.2.2 TRANSFER OF STORMWATER RUNOFF BETWEEN WATERSHEDS Stormwater runoff from a development should remain within the existing watershed. §502.2 of the WMO prohibits development that results in the transfer of stormwater between watersheds, unless the transfer does not violate the provisions of §501.1. For this provision, the following items apply: • “Watershed” refers to the tributary area to a body of water; and • “Transfer of stormwater” refers to the diversion of stormwater runoff or stream flow from one watershed to a different watershed by overland flow paths or storm sewer systems. Prior to development, some sites may contain a ridgeline that results in the site being tributary to multiple watersheds. To demonstrate compliance with §502.2, the existing and proposed site must be evaluated. The proposed grading plan should preserve the natural drainage boundaries that define the watersheds. The following resources are available to evaluate the site: • The drainage boundaries of the watersheds are available online and can be viewed with the District’s Stormwater Inundation Mapping Application at the following link: gispub.mwrd.org/swima • USGS topographic maps are available online and can be viewed with the topoView application at the following link: ngmdb.usgs.gov/topoview/ • Cook County topography maps are available online and can be viewed with the CookViewer Map Application at the following link: maps.cookcountyil.gov/cookviewer/ 10/7/2019 5.2 RUNOFF REQUIREMENTS PAGE 5-5 WATERSHED MANAGEMENT ORDINANCE TECHNICAL GUIDANCE MANUAL If the proposed development revises the watershed drainage boundaries, the applicant must submit plans and calculations to demonstrate it will not increase flood elevations, velocities or flow rates, or decrease flood conveyance capacity to upstream, downstream, or adjacent property. Computation of flows must be completed using the methodology required for major stormwater systems. 5.2.3 CONCENTRATED DISCHARGES §502.3 of the WMO requires concentrated discharges from stormwater facilities enter conveyance systems that
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